A panel dataset should have data on n cases, over t time periods, for a total of n × t observations. Data like this is said to be in long form. In some cases your data may come in what is called the wide form, with only one observation per case and variables for each different value at each different time period. To analyze data like this in Stata using commands for panel data analysis, you need to first convert it to long form. This can be done using Stata's reshape command

reshape long EXPOSURE, i(GROUP) j(year)  
(note:  j = 2000 2001 2002)

Data                               wide   ->   long
-----------------------------------------------------------------------------
Number of obs.                        200 ->       600

Number of variables                   4   ->       3
j variable (3 values)                     ->   year
xij variables:
 EXPOSURE2000 EXPOSURE2001 EXPOSURE2002 ->       EXPOSURE---------------------

long tells reshape that we want to go from wide to long

EXPOSURE tells Stata that the stem of the variable to be converted from wide to long is EXPOSURE

i(GROUP) option tells reshape that GROUP is the unique identifier for records in their wide format

j(year) tells reshape that the suffix of faminc (i.e., 2000 2001 2002) should be placed in a variable called year

The reshape wide command puts the data back into wide format